1. Field of the Invention
The invention relates to a method for producing articles by reactive infiltration and to articles so produced. More particularly, the invention relates to a method for producing articles by directional reactive infiltration and to articles so produced.
2. Description of the Prior Art
Refractory compound materials include intermetallic compounds such as, for example, NiAl, Ni.sub.3 Al, Fe.sub.3 Al, FeAl, Ti.sub.3 Al and TiAl as well as silicide, oxide, boride, carbide and nitride compounds. Such compounds are usually produced using one of several techniques, including melting, casting, powder metallurgy processing techniques and infiltration processes. Such compounds are desirable for use in hostile environments because of their high melting points, as well as corrosion and oxidation resistance. However, the performance of these materials is often marred by brittleness near room temperature, and service temperatures which are low by comparison with the melting points of the compounds. In order to improve these properties, refractory second phase reinforcements are often added to the refractory compound materials which complicates processing of these materials.
Reactive powder metallurgy techniques wherein elemental powders of the elements making up the compound are mixed and compacted to form powder compacts which are then heated to a temperature at which the elemental powders react vigorously to form the desired compound are especially attractive for the processing of these refractory materials. Elemental powders are readily commercially obtainable and easy to compact.
In reactive powder metallurgy processing, heating of reactant materials to high temperature is avoided and, instead, this processing makes it possible to take advantage of the high enthalpic stability of the desired product compound to chemically drive synthesis of the compound with an accompanying release of heat. According to reactive powder metallurgy techniques, the elemental powder compacts are usually heated to the lowest temperature at which a liquid phase is formed, so that liquid phase densification occurs simultaneously with the rapid heating resulting from exothermic formation of the desired compound. The chemical composition of the resulting material can be tailored by adjusting the ratio of elemental powders making up the powder compact.
Disadvantages associated with reactive powder metallurgy techniques include a frequent need for further densification of the refractory compound material by application of external pressure, either during or after the reaction, in order to eliminate undesirable porosity in the refractory material. Generally, this external pressure is applied using expensive processing steps such as hot isostatic pressing. It is often difficult to control the powder reaction kinetics and fiber or whisker reinforcements, included to produce a reinforced composite refractory material. Reinforcements often break during powder compaction, making reactive powder metallurgy techniques unattractive for reinforced refractory compound fabrication.
A second processing technique useful for refractory compound material fabrication, including reinforced composite refractory compound materials, is infiltration processing. Infiltration processing involves the injection of a liquid infiltrant into the interstices of a preform, which can include fiber, whisker or particle second phase reinforcements. There exist several types of infiltration processes including (1) pressureless infiltration, (2) pressure infiltration and (3) reactive infiltration processes. There is frequently overlap among these infiltration processes, and, for example, a reactive infiltration process may be performed with pressurized or unpressurized infiltrant.
Pressureless infiltration techniques include the Lanxide process, for example U.S. Pat. No. 4,904,446 to White et al., issued Feb. 27, 1990.
More commonly, pressure is applied to force a liquid infiltrant into the porous preform. Pressure infiltration processes include squeeze casting, for example Fukunaga et al., Journal of Materials Science Letters, 9 (1990) 23-25, variations of die-casting and pressurized gas driven infiltration. These techniques have been applied to high-melting point materials, including nickel aluminide and titanium aluminide, for example Nourbakhsh et al., Metallurgical Transactions A, 20A, (1989) 2159-2166. Applying pressure to the liquid infiltrant makes the process faster, improves the microstructure of the material produced, and minimizes undesirable chemical reactions between the preform and the infiltrant.
In reactive infiltration processes, a compound is formed by infiltration of a porous, solid preform with a liquid infiltrant which reacts with the preform to form a desired refractory compound material. In conventional reactive infiltration processes, the reaction progresses globally in a direction parallel to the direction of liquid infiltrant flow. Using reactive infiltration techniques, a composite can be produced, either by incorporating inert second phase reinforcements in the preform or by forming a second reinforcement phase as the result of an in-situ reaction. Reactive infiltration has been used successfully to produce ceramic materials such as silicon carbide, for example U.S. Pat. No. 4,737,476, to Hillig, issued Apr. 12, 1988.
Difficulties encountered in reactive infiltration processing include an inability to control the reaction rate such that it remains sufficiently slow to permit complete infiltration of the preform, but sufficiently high to form the desired compound material in a reasonable process time. Flow of liquid infiltrant into the preform becomes blocked when the compound product of the chemical reaction between the infiltrant and preform, which is most often a solid, grows to such an extent where the liquid infiltrant first enters the preform so that flow of the liquid infiltrant is blocked beyond that point, precluding the filling of pores resulting from reaction or solidification-induced shrinkage downstream from the liquid infiltrant entry point, or even blocking complete preform infiltration.
Another technique for the fabrication of refractory compound materials is self-propagating synthesis (SHS), also known as self-propagating high-temperature synthesis, wherein a strongly exothermic reaction, i.e. combustion, is initiated among powders in a powder compact as described by Wrzesinski et al., Journal Of Materials Science Letters, 9, (1990) 432-435 and Merzhanov, Combustion And Plasma Synthesis Of High-Temperature Materials, Z. A. Munir, J. B. Holt, (Eds.) VCH, New York (1990) 1-53. Here, as in other powder processes, volume changes may leave pores.
Thus, there exists a need for a versatile, near-net-shape and economical process appropriate for composite fabrication which results in production of materials which do not require further densification and which allows precise control of the reaction rate between the liquid infiltrant and the preform so that the product material is dense and fully reacted.